1. Field of the Invention
The present invention relates to an avalanche photodiode, more particularly to a technology for raising productivity.
2. Description of the Background Art
A diode structure actually used and including the aforementioned avalanche photodiode is of a planer type structure in which either one conductive type of pn-conductive types is formed as a selective area, while having long-term reliability. Particularly, a diode using a compound semiconductor lattice-matched to InP for use in optical communications and the like utilizes, as the aforementioned selective area, a p-type conductive area in which Zn is diffused thermally in the InP having a low conductivity as a p-conductive type impurity. The avalanche photodiode is further provided with a p-type conductive area called a guard ring on its outer periphery in order to prevent a local voltage drop called edge break-down due to concentration of an electric field to the periphery of the p-type conductive area. This p-type conductive area is formed by ion injecting Be (see, for example, Japanese Patent Application Laid-Open No. 58-48478 (1983) (FIG. 2)) or diffusing Zn thermally (see, for example, U.S. Pat. No. 4,857,982 (FIG. 2)).
Japanese Patent Application Laid-Open No. 2004-200302 (see FIG. 1) discloses an avalanche photodiode in which at least an (i-type) AlInAs avalanche intensifying layer, a GaInAs light absorption layer and an InP window layer are overlaid in order as a semiconductor on an InP substrate so as to form the p-type conductive area in the InP window layer. The avalanche photodiode having a high reliability and a low dark current can be easily achieved without necessity of manufacturing the guard ring because an electric field intensity in the InP window layer can be weakened by disposing the AlInAs avalanche intensifying layer below the GaInAs light absorption layer. In the meantime, according to Japanese Patent Application Laid-Open No. 2004-200302 (see FIG. 1), the p-type conductive area is provided by diffusing Zn thermally.
Because a conventional avalanche photodiode is structured as described above, the diffusion depth of Zn may determine the thickness of i-layer of a pin diode structure and further determine the intensifying layer thickness of the avalanche photodiode. Accordingly, the Zn diffusion depth needs to be controlled precisely in order to obtain desired device characteristics such as a frequency characteristic, an electric characteristic and a capacity. However, it is difficult to control the thermal diffusion depth of an impurity accurately with excellent reproducibility without any dispersion of accuracy in its wafer plane. Therefore, there exists such a problem that the yield drops thereby reducing productivity.